1. Field of the Invention
The present invention relates to an electronic circuit. More particularly, the invention relates to techniques for noise cancellation in an electronic circuit.
2. Background of the Related Art
There are many applications that benefit from a stable low jitter oscillator. For example, in an application which uses a high frequency clock to measure the periods of a low frequency asynchronous oscillator, the stability of the high frequency clock directly impacts the final result.
One means to generate a high frequency oscillator is shown in FIG. 1. This circuit operates by charging a capacitor (103) with a constant current source Ichg (101) into a timing capacitor Cosc (103) (note: assume the comparator output is LOW to start). When the voltage across the capacitor (103) exceeds the threshold voltage Vth (104), the output (106) of the comparator (107) goes high forcing switch (102) to close. Closing switch (102) causes Cosc (103) to discharge causing the comparator output (106) to go LOW. When the comparator output (106) is LOW the capacitor Cosc (103) is once again allowed to charge, and the oscillation repeats. The comparator output (106) produces a string of short pulses while the positive input terminal (108) to the comparator (107) sees a saw tooth waveform.
It should be apparent to one skilled in the art that increasing Vth (104) will decrease the oscillator frequency, as it takes longer to charge Cosc (103), while decreasing Vth (104) will increase the oscillator frequency. Electronic noise in either Vth (104), Ichg (101) or the input referred noise to the comparator (107) will create changes in the frequency (i.e. phase noise) of the oscillator.
For the example described above (i.e. using a high frequency oscillator to count a fixed duration of a lower frequency oscillator), 1/F noise is of particular concern. 1/F noise has the characteristic of increasing in magnitude as the noise is examined at lower frequencies. Thus counting a long duration of the low frequency oscillator increases the need for stabilizing the high frequency oscillator against 1/F noise. The longer the time duration to be counted the lower in frequency is the 1/F noise and the more problematic it is in maintaining the frequency stability of the high frequency oscillator over that long time duration.
1/F noise is also problematic for measuring short durations of the low frequency oscillator in that repeated measurements taken with long intervals between measurements (e.g., seconds or days) will cause non-repeatable results. There are several sources for 1/F noise in the above circuit, for example, comparator input referred noise, Vth source and Ichg current. The examples disclosed here eliminate all of these sources of 1/F noise from contributing to changes in the oscillator frequency.